Non-aqueous secondary batteries, such as lithium ion secondary batteries, have been widely used as power supply for portable electronic devices such as lap-top computers, mobile phones, digital cameras, and camcorders. Further, in recent years, since these batteries have high energy density, application of these batteries to automobiles and the like has also been studied.
In conjunction with reductions in size and weight of portable electronic devices, the outer casing of non-aqueous secondary batteries has been simplified. Recently, as the outer casing, a battery can made of an aluminum can has been developed in place of the battery can made of stainless steel that was previously used, and further, currently, a soft pack outer casing made of an aluminum laminate pack has also been developed.
In the case of a soft pack outer casing made of aluminum laminate, since the outer casing is soft, a space may form between an electrode and a separator in conjunction with charging and discharging. This is one of factors contributing to deterioration of the cycle life, and therefore, uniform retention of the adhesive property of adhered portions of electrodes, separators, or the like, is a significant technical problem.
As techniques relating to the adhesive property, various techniques for enhancing the adhesion between an electrode and a separator have been proposed. As one of such techniques, a technique of using a separator in which a porous layer (hereinafter also referred to as an “adhesive porous layer”) using a polyvinylidene fluoride resin is formed on a polyolefin microporous membrane, a conventional separator, has been proposed (see, for example, Patent Documents 1 to 4). The adhesive porous layer functions as an adhesive that favorably joins the electrode and the separator together, in a case in which an adhesive porous layer and an electrode is disposed adjacently in layers and subjected to compression bonding or heat pressing. Accordingly, the adhesive porous layer contributes to improvement of the cycle life of a soft pack battery.
In a separator having a polyolefin microporous membrane and an adhesive porous layer formed thereon as described above, from the viewpoint of achieving both ion permeability and ensuring sufficient adhesive property, a new technical proposal focusing upon the porous structure and thickness of a polyvinylidene fluoride resin layer, and a new technical proposal that uses two kinds of polyvinylidene fluoride resins in combination, have been made.
Further, in the case of producing a battery using a conventional metal can outer casing, electrodes and a separator are disposed adjacently in layers and wound to produce a battery element, and this element is enclosed in a metal can outer casing together with an electrolyte, thereby producing a battery. Meanwhile, in the case of producing a soft pack battery using a separator having the adhesive porous layer described above, a battery element is produced in a manner similar to that in the production of a battery having a metal can outer casing as described above, after which this battery element is enclosed in a soft pack outer casing together with an electrolyte, and thereafter, is finally subjected to a heat pressing process, thereby producing a battery. Accordingly, in the case of using such a separator, a battery element can be produced in a manner similar to that in the production of a battery having a metal can outer casing as described above. This is advantageous in that it is not necessary to greatly change the production process from that for conventional batteries having a metal can outer casing.
Patent Document 1: Japanese Patent No. 4127989
Patent Document 2: Japanese Patent No. 4490055
Patent Document 3: Japanese Patent No. 4109522
Patent Document 4: Japanese Patent No. 4414165